A number of E. coli bacteriophage (phi X174, S13, P2, T4 and lambda) code for a protein which is required for, and appears to, initiate host cell lysis. In bacteriophage lambda this protein is the product of a late gene called the S gene. A number of different studies suggest that this protein functions by increasing the permeability of the E. coli inner membrane. The S protein will be identified by comparing the labelled proteins present in the inner membranes of UV irradiated cells infected with either lambda R phage or lambda RS phage where the mutant genes contain chain termination mutations. Once the S protein is identified with a particular band after SDS polyacrylamide gel electrophoresis of the inner membrane from lambda infected cells it will be extracted from the membrane with a nonionic detergent and purified to homogeneity. The purified protein will be characterized and reconstituted into phospholipid vesicles to study its effects on the permeability of the vesicles. The S protein from a temperature sensitive mutant will also be purified and used in the reconstitution to insure that any observed permeability changes are due to the S protein. Finally the exact role of the S protein in causing cell lysis will be studied.